A measurement method of detecting an agglutination reaction has been conventionally and widely-used (applied) in a method such as a plate agglutination test. In particular, an immune agglutination method utilizing an antigen-antibody reaction has been mainly used because of the excellence in sensitivity, specificity, and reproducibility thereof.
Recently, the use of an insoluble carrier particle sensitized with an antibody which specifically recognizes a target ligand as an antigen and a widely-used automatic analyzer has enabled simple measurement of the ligand with higher sensitivity. Therefore, the immune agglutination method is widely-used in a clinical test and the like. The immune agglutination method is a method of measuring a concentration of a target ligand performed by allowing an insoluble carrier particle to agglutinate on the basis of the antigen-antibody reaction and optically measuring a degree of the obtained agglutination. As the insoluble carrier particle, a latex is commonly used. In this case, the method is particularly called a latex turbidimetric immunoassay (hereinafter abbreviated as an LTIA method).
It has been known, in the LTIA method, addition of an agglutination accelerator such as sodium chloride or guanidines accelerates formation of immune agglutination and improves measurement sensitivity (Patent Document 1).
However, coexistence of those agglutination accelerators with antibody-sensitized latexes resulted in nonspecific aggregation of the antibody-sensitized latexes (also called spontaneous aggregation) irrelevant to an immune reaction. There was a problem that after a long-term storage in the coexistence state, the particles were finally precipitated. Therefore, as a measurement reagent for the LTIA method, a two-shot reagent format is commonly used. The antibody-sensitized latex and the agglutination accelerator are separately prepared as constituent reagents and mixed together when measurement is performed.
In Patent Document 2, a method of dissolving aminosulfonate into a reagent to disperse a latex reagent is disclosed, because aminosulfonate suppresses nonspecific aggregation derived from a component of a measurement sample in a latex agglutination reaction, and is excellent in storage stability.
In addition, Patent Document 3 discloses a stabilization method, in which amino acid ester or polyamine is allowed to coexist in a latex reagent to enable stabilization of proteins, whereby nonspecific aggregation of proteins each bonded to a latex with one another is suppressed.
However, those documents only describe stabilization of the reagent that the nonspecific aggregation of latexes is suppressed, and do not describe an effect of accelerating an agglutination reaction.
Besides, even in the case where sodium chloride or guanidines was used as a reaction accelerator, satisfactory measurement sensitivity was not necessarily obtained when measurement time was limited as in the case of measurement using an automatic analyzer. Therefore, provision of a reaction accelerator having a higher effect has been expected.    Patent Document 1: JP 11-344493 A    Patent Document 2: JP 07-229900 A    Patent Document 3: JP 2004-108850 A